1. Field of the Invention
The present invention relates to an image forming apparatus which uses an electrophotographic system.
2. Description of the Related Art
In an image forming apparatus such as a copier, a laser printer, or a facsimile, a film heating-type fixing apparatus,. which uses a ceramic heater as a heat source, is widely used as a fixing apparatus that heats and fixes a toner image formed on a recording sheet. The heater is connected to an AC power supply via a switching element such as a triac or a mechanical switch element such as a relay. In general, power is supplied to the heater by turning the switching element on and off so as to maintain the temperature detected by a temperature detection element disposed near the heater. The on/off control is performed based on a predetermined current waveform pattern. This waveform pattern is determined according to phase control that controls the energization ratio in a half-wave of an AC power supply, wave-number control that uses a predetermined number of successive half-waves of an AC power supply as one control cycle and controls the number of half-waves corresponding to an energization period in one control cycle, or a combination of the phase control and wave-number control. These control methods are determined by taking flicker and harmonic currents into consideration.
Here, flicker is a phenomenon in which a lighting equipment flicker due to the fluctuation of the voltage of an AC power supply under the influence of a load current fluctuation in an electric equipment connected to the same AC power supply as the lighting equipment and an output impedance of the AC power supply. A perceptibility short term (Pst: short-term flicker value), which is a statistically calculated index, is frequently used as a flicker level. International Electrotechnical Commission (IEC) defines a standard Pst value (see IEC 61000-3-3). The larger the voltage fluctuation, the larger (worse) the Pst. Moreover, the Pst is weighted according to the frequency and increases particularly when a voltage fluctuation occurs near 10 Hz, where human perceptibility is maximized. On the other hand, standard values for 2nd-order to 40th-order harmonic currents are defined using an AC power supply as a fundamental wave (see IEC 61000-3-2). The larger the degree of distortion from a sinusoidal wave, of a current waveform from the AC power supply, the more likely the harmonic current is to occur.
Thus, the phase control in which energization is performed for every wave is advantageous in suppressing flicker since a voltage fluctuation at such a low frequency as 10 Hz rarely occurs, but is disadvantageous in suppressing harmonic currents since the degree of distortion from a sinusoidal wave is large. On the other hand, wave-number control in which a current waveform pattern is repeated in one control cycle is disadvantageous in suppressing flicker since a low-frequency voltage fluctuation is likely to occur, but is advantageous in suppressing harmonic currents since energization is not performed in the middle of a half-wave. As above, although flicker and the harmonic current are generally in a trade-off relation with respect to the current waveform pattern, the current waveform pattern needs to be set so as to satisfy the flicker and harmonic current standards. In recent years, since image forming apparatuses have been operating at higher speed and requiring larger power, and the resistance value of the heater has been decreasing further, it has become difficult to set the current waveform pattern that satisfies both standards.
To cope with this, a method for satisfying the flicker and harmonic current standards by dividing the heater into a plurality of parts, connecting the parts in parallel, and forming a switching element in each part is proposed. That is, this method involves decreasing the harmonic current value by performing phase control so that energization of a plurality of heaters does not start at the same time-point and suppressing flicker by performing wave-number control so that the total voltage fluctuation in the plurality of heaters in one control cycle decreases. However, this method may increase the circuit size and incurs a large increase in the cost.
Moreover, a method of suppressing harmonic currents, by arranging an active filter and a high-frequency coil in an AC/DC power supply circuit unit that generates a voltage for a drive member, such as a motor, and a voltage for a control unit so that a current waveform from of the AC power supply approaches a sinusoidal wave, is often used. However, since the active filter circuit is complex and includes a large number of components and the high-frequency coil is large and heavy, any of the above-mentioned configurations results in a large increase in the cost.
Moreover, various control methods for changing the current waveform pattern according to an operating condition of an image forming apparatus are proposed. For example, a control method of determining a voltage area (100V area or 200V area) based on a voltage of an AC power supply in an image forming apparatus, which uses a universal AC/DC power supply, and selecting phase control or wave-number control based on the determination result, is proposed. That is, phase control that is advantageous in suppressing flicker is selected for the 100V area since the 100V area uses a large load current as compared to the 200V area and the voltage fluctuation of the AC power supply is large. On the other hand, wave-number control that is advantageous in suppressing harmonic currents is selected for the 200V area since the 200V area uses a higher AC power supply voltage as compared to the 100V area. Further, a control method of switching between phase control and wave-number control according to print conditions, such as a process speed or a control target temperature, is also proposed. Further, Japanese Patent Application Laid-Open No. 2008-40072 proposes a control method of detecting the intensity of illumination of the surroundings using an illuminometer and switching between phase control and wave-number control based on the detection result. The illuminometer detects flicker in the surroundings, and phase control is performed when the flicker is large, whereas wave-number control is performed when the flicker is small.
The output impedance of the AC power supply has correlation with the flicker Pst and the harmonic current. In general, the output impedance of an AC power supply includes the output impedance of a transformer on the electric pole, the line impedance of a lead-in wire extending from the transformer on the electric pole to an outlet via a distribution board, and the line impedance of a power supply cable extending from the outlet to an inlet portion of the image forming apparatus. The output impedance of the AC power supply is different depending on the output impedance of the transformer on the electric pole and the material, the thickness, the length, and the wiring method of the lead-in wire and the power supply cable.
FIG. 5 illustrates the relation among an output impedance, the flicker Pst, and the harmonic current. The horizontal axis represents the absolute value |Zout (50 Hz)| of an output impedance Zout at the frequency 50 Hz of an AC power supply and the vertical axis represents a flicker Pst and a harmonic current. In the graph, a solid line indicates the flicker Pst and the broken line indicates the harmonic current. From FIG. 5, it can be understood that the larger the output impedance of the AC power supply, the larger the flicker level becomes. This is because the voltage fluctuation increases due to the output impedance. Moreover, it can be understood that the smaller the output impedance of the AC power supply, the larger the harmonic current becomes. This is because the smaller the output impedance, the larger the harmonic current flowing from the AC power supply to the image forming apparatus.
The IEC standards define flicker Pst as being measured at an output impedance of 0.4+j0.25Ω and define harmonic current as being measured at an output impedance of approximately 0 (that is, an AC power supply having a sufficiently small output impedance is used and no additional impedance is inserted). As indicated by reference numerals 501 and 502 in FIG. 5, this means that both the flicker Pst and the harmonic current are measured in very unfavorable conditions. In other words, this means that the flicker Pst and harmonic current standards need to be satisfied for an AC power supply having a wide range of output impedances of 0 to 0.4+j0.25Ω.
In contrast, in the method of Japanese Patent Application Laid-Open No. 2008-40072, although power control based on the output impedance can be realized to some extent by switching the control based on the detected flicker, it is necessary to add the illuminometer, which results in a considerable increase in the cost and an increase in the arrangement space.